The costs of importing timber from USA dealers, sawmills, or even individuals varies really widely which makes it very hard to give any realistic or accurate guide. If you have an idea of the hardwood you would like to use for your project then you will find that in the US there are suppliers which specialize in certain species. If you want really good, interestingly figured maple, cherry, walnut, or central/south American exotics then there are companies which specialize in all of these. So it is probably best to decide what you want in terms of wood species, figure how much of it you need, what thickness you need, whether you want it dry, planed or sawn, then look online for dealers which are advertising stocks of what you seek. Once you find someone then email them asking if they well ship overseas. Actually, you could also look on American ebay for timber (lumber) - you may be surprized by the quality of what is offered.
In the USA you will find dealers specializing in huge table top sizes of walnut, even dealers specializing in a particular type of walnut such as claro or paradox/Bastogne (
http://siouxdentalhealth.com/home),
http://www.curlymaple.com/ has the most gorgeous collection of amazingly figured curly maple, Hearne Hardwoods have an amazing collection of beautiful stuff, Diamond Tropical Hardwoods do superb exotics such as cocobolo and goncalo alves - exquisite timber. I have bought quite a lot of timber from Kevin at Diamond Tropical over the years and have always been thoroughly delighted with what he sends me. Other dealers specialise in mesquite, koa, parota, redwood (rare now but blooming gorgeous if you can get first growth) ... the list is practically unending.
Be warned, once you start looking at what wonders are available from across the pond, you will be hooked. I am!
If you like I can post some images of furniture I have made from wood I imported from America. my designs may not be your cup of tea but the woods I have had the pleasure of working with are impressive, and nigh on impossible to get here in the UK at sensible money.
Spoil yourself by visiting
www.ancientwood.com - a north east USA based dealer in kauri - the world's oldest, most fascinating, and perhaps most beautiful wood - think shimmering cognac. Robert, the boss of Ancientwood is another really helpful guy who will happily ship his timber to you, but be prepared to remortgage your home - the best is not cheap!
C.